In a sound reproduction environment, various factors may serve to degrade the quality of reproduced sound as perceived by a listener. Such factors distinguish the sound reproduction from that of an original sound stage. One such factor is the location of loudspeakers in a sound stage which, if inappropriately placed, may lead to a distorted sound-pressure response over the audible frequency spectrum. The placement of loudspeakers also affects the perceived width of a soundstage. For example, loudspeakers act as point sources of sound limiting their ability to reproduce reverberant sounds that are easily perceived in a live sound stage. In fact, the perceived sound stage width of many audio reproduction systems is limited to the distance separating a pair of loudspeakers when placed in front of a listener. Another factor degrading the quality of reproduced sound may result from microphones which record sound differently from the way the human hearing system perceives sound. In an attempt to overcome the factors which degrade the quality of reproduced sound, countless efforts have been expended to alter the characteristics of a sound reproduction environment to mimic that heard by a listener in a live sound stage.
Some efforts at stereo image enhancement have focused on the acoustic abilities and limitations of the human ear. The human ear's auditory response is sensitive to sound intensity, phase differences between certain sounds, the frequency of the sound itself, and the direction from which sound emanates. Despite the complexity of the human auditory system, the frequency response of the human ear is relatively constant from person to person.
When sound waves having a constant sound pressure level across all frequencies are directed at a listener from a single location, the human ear will react differently to the individual frequency components of the sound. For example, when sound of equal sound pressure is directed towards a listener from in front of the listener, the pressure level created within the listener's ear by a sound of 1000 hertz will be different from that of 2000 hertz.
In addition to frequency sensitivity, the human auditory system reacts differently to sounds impinging upon the ear from various angles. Specifically, the sound pressure level within the human ear will vary with the direction of sound. The shape of the outer ear, or pinna, and the inner ear canal are largely responsible for the frequency contouring of sounds as a function of direction.
The human auditory response is sensitive to both azimuth and elevation changes of a sound's origin. This is particularly true for complex sound signals, i.e., those having multiple frequency components, and for higher frequency components in general. The variance in sound pressure among the frequency components within the ear is interpreted by the brain to provide indications of a sound's origin. When a recorded sound is reproduced, the directional cues to the sound's origin, as interpreted by the ear from sound pressure information, will thus be dependent upon the actual location of loudspeakers that reproduce the sound.
A constant sound pressure level, i.e., a “flat” sound pressure versus frequency response, can be obtained at the ears of a listener from loudspeakers positioned directly in front of the listener. Such a response is often desirable to achieve a realistic sound image. However, the quality of a set of loudspeakers may be less than ideal, and they may not be placed in the most acoustically-desirable location. Both such factors often lead to disrupted sound pressure characteristics. Sound systems of the prior art have disclosed methods to “correct” the sound pressure emanating from loudspeakers to create a spatially correct response thereby improving the resulting sound image.
To achieve a more spatially correct response for a given sound system, it is known to select and apply head-related-transfer-functions (HRTFs) to an audio signal. HRTFs are based on the acoustics of the human hearing system. Application of an HRTF is used to adjust the amplitudes of portions of the audio signal to compensate for spatial distortion. HRTF-based principles may also be used to relocate a stereo image from non-optimally placed loudspeakers.
A second type of deficiency often occurs because it is difficult to adequately reproduce low-frequency sounds such as bass. Various conventional approaches to improving the output of low-frequency sounds include the use of higher quality loudspeakers with greater cone areas, larger magnets, larger housings, or greater cone excursion capabilities. In addition, conventional systems have attempted to reproduce low-frequency sounds with resonant chambers and horns that match the acoustic impedance of the loudspeaker to the acoustic impedance of free space surrounding the loudspeaker.
Not all systems, however, can simply use more expensive or more powerful loudspeakers to reproduce low-frequency sounds. For example, some conventional sound systems such as compact audio systems and multimedia computer systems rely on small loudspeakers. In addition, to conserve costs, many audio systems use less accurate loudspeakers. Such loudspeakers typically do not have the capability to properly reproduce low-frequency sounds and consequently, the sounds are typically not as robust or enjoyable as systems that more accurately reproduce low-frequency sounds.
Some conventional enhancement systems attempt to compensate for poor reproduction of low-frequency sounds by amplifying the low-frequency signals prior to inputting the signals into the loudspeakers. Amplifying the low-frequency signals delivers a greater amount of energy to the loudspeakers, which in turn, drives the loudspeakers with greater forces. Such attempts to amplify the low-frequency signals, however, can result in overdriving the loudspeakers. Unfortunately, overdriving the loudspeakers can increase the background noise, introduce distracting distortions, and damage the loudspeakers.
Still other conventional systems, in an attempt to compensate for the lack of the lower-frequencies, distort the reproduction of the higher frequencies in ways that add undesirable sound coloration.
A third difficulty arises because sounds emanating from multiple locations are often not properly reproduced in an audio system. One approach directed to improving the reproduction of sound includes surround sound systems that have multiple recording tracks. The multiple recording tracks are used to record the spatial information associated with sounds that emanate from multiple locations.
For example, in a surround sound system, some of the recording tracks contain sounds that originate from in front of the listener, while other recording tracks contain sounds, which originate from behind the listener. When multiple loudspeakers are placed around the listener, the audio information contained in the recording tracks makes the produced sounds appear more realistic to the listener. Such systems, however, are typically more expensive than systems which do not use multiple recording tracks and multiple speaker arrangements.
To conserve costs, many conventional two-speaker systems attempt to simulate a surround sound experience by introducing unnatural time-delays or phase-shifts between left and right signal sources. Unfortunately, such systems often suffer from unrealistic effects in the reproduced sound.
Other known sound enhancement techniques operate on what are called “sum” and “difference” signals. The sum signal, which is also called the monophonic signal, is the sum of the left and right signals. This can be conceptualized as adding or combining the left and right signals (L+R).
The difference signal, on the other hand, represents the difference between the two left and right audio signals. This is best conceptualized as subtracting the right signal from the left signal (L-R). The difference signal is also often called the ambient signal.
It is known that modifying certain frequencies in the difference signal can widen the perceived sound projected from the left and right loudspeakers. The widened sound image typically results from altering the reverberant sounds, which are present in the difference signal.
The circuitry that generates the sum and difference signals, however, generates the sum and difference signals by processing of the left and right input signals. Furthermore, once the circuitry generates the sum and difference signals, additional circuitry then separately processes and recombines the sum and difference signals in order to produce an enhanced sound effect.
Typically, the creation and processing of the sum and difference signal are accomplished with digital signal processors, operational amplifiers and the like. Such implementations usually require complicated circuitry that increases the cost of such systems. Thus, despite the contributions from the prior art, there exists a need for a simplified audio enhancement system that reduces costs associated with producing an enhanced listening experience.